A typical organic light-emitting device includes an anode, an active light-emitting zone comprising one or more electroluminescent organic material(s), and a cathode. One of the electrodes is optically transmissive while the other one is optically reflective. The function of the anode is to inject positively charged particles referred to as holes into the light-emitting zone, and that of the cathode is to inject electron into the emission zone. A process involved in the recombination of the electron and the hole leads to the creation of light wave. The light wave is escaped through one of the aforementioned electrodes.
U.S. Pat. No. 4,356,429 discloses inserting a hole-transport organic layer between the anode and the emission zone, and an electron-transport organic layer between the cathode and the emission zone.
It is well documented that many different types of organic molecules provide an excellent medium for the transport of holes but unfortunately they are very poor in regard to the transport of electrons. It is critical to have balance electron and hole density in the emission zone to obtain optimal device performance. In order to enhance the electron injection, low work function metals such as Ca and Mg, which provide excellent energy band-matching to that of the lowest unoccupied molecular orbital (LUMO), have been selected as cathode materials. However, the low work function metals are highly reactive that leads to fragmentation of the organic molecules when vapour phase metal atoms strike on the organic film surface [A. Turak, D. Grozea, X. D. Feng, Z. H. Lu, H. Aziz, and A. M. Hor, “Metal/Alq Interface Structures”, Appl. Phys. Left. 81, 766 (2002).]. This limits the ability to use low work function metals as cathode. In general the cathode interface stability suffers from the fact that organic and polymeric materials contain more than one element which are very reactive. Some elements such as oxygen readily react with elements making up the cathode to form interfacial oxides, which results in electrical degradation [A. Turak, D. Grozea, X. D. Feng, Z. H. Lu, H. Aziz, and A. M. Hor, “Metal/Alq Interface Structures”, Appl. Phys. Lett. 81, 766 (2002)].
As a family member of naturally occurring allotropes of carbon, fullerene materials are known for their robust structures and superior charge transport properties. U.S. Pat. No. 5,861,219 discloses the use of fullerenes as a dopant added to a host metal complex of 5-hydroxy-quinoxaline used in organic light emitting diodes. The host metal complex of 5-hydroxy-quinoxaline is contained in the electroluminescent layer which forms the emission zone in the structure. United States Patent Publication US 2002/0093006 A1 discloses the use of a fullerene layer as the light emissive layer in an organic light emitting diode structure.
United States Patent Publication US 2003/0042846 A1 discloses the use of a fullerene layer as an electron acceptor layer in organic photovoltaic devices.
Japan Patent 3227784 and Japanese patent application 04-144479 disclose the use of fullerenes as a hole transport layer.
U.S. Pat. No. 5,171,373 discloses the use of fullerenes in solar cells. U.S. Pat. No. 5,759,725 discloses the use of fullerenes in photoconductors.
The use of fullerenes as an interface layer between the hole transport layer and the light emission layer has been disclosed by Keizo Kato, Keisuke Suzuki, Kazunari Shinbo, Futao Kaneko, Nozomu Tsuboi, Satosh Kobayashi, Toyoyasu Tadokoro, and Shinichi Ohta, Jpn. J. Appl. Phys. Vol. 42, 2526 (2003).
U.S. Pat. No. 5,776,622 issued to Hung et al. discloses an electroluminescence device including an anode, cathode and EL layer, in which the cathode layer contacts the EL layer and includes a fluoride layer in direct contact with the EL layer and a conductive layer in direct contact with the fluoride layer.
The light output of a typical OLED involves two-coupled light waves, one propagates directly through of the light-transmissive electrode, and the other reflected back from the reflective electrode and then propagate through the light-transmissive electrode. These two light waves have approximately the same amplitude and the optical interference of these two light beams dictates the total amplitude and spectral profile of the final output light waves.
It would be very advantageous to provide an organic-based electroluminescence device which provides better electronic transport in the electronic transport layer and better transport across the electronic transport layer from the cathode electrode layer to the light emission zone.